Ultrasonic flow sensors have been employed for a number of years for performing intraoperative or extracorporeal blood flow measurements. Intraoperative flow measurements are typically conducted to monitor dynamic parameters of blood flow in various vessels during vascular, cardiac, transplant, plastic and reconstructive surgery. Flow sensors used for measurements through blood vessels are commonly referred to as perivascular probes. Extracorporeal blood flow measurements are made externally of the patient during procedures in which the blood is removed from the patient for treatment, such as for example, ECMO, hemodialysis, CP bypass and CAVH procedures. Flow sensors used for these purposes on tubing are commonly referred as clamp-on sensors.
While existing ultrasonic blood flowmeters may appear as well suited to perivascular use, this is often not in fact the case. A particular difficulty arises in connection with the coupling of an ultrasound transducer with a vascular vessel. Ultrasound/tissue coupling conventionally involves a water bath, which is often impractical, or a water-soluble gel, which is inappropriate in a wet surgical field.
Prior designs of flow sensors have also included clamping configurations for deforming the conduit wall into a predetermined cross-section of a measuring channel. It is often unacceptable to substantially deform a conduit inside the measuring channel, e.g. in case of blood flow measurements on a human vessel that may be calcified. The squaring could dislodge parts of the calcified plaque which would then become lodged in the microvasculature of the patient and could cause an infarct. If a biocompatible extracorporeal blood line tubing is “squared” inside the flow sensor, the tubing deformation may raise the concern that the sudden change in flow profile could induce thrombosis, or that the deformation itself could damage the inside anti-thrombogenic coating of specialty tubing. Also, the distortion of the conduit alters the average flow and flow profile within the conduit, which constitutes an error in the measurement, in all the situations where the user would want to know the flow through the conduit in its undisturbed state.
Alternatively, the prior flow sensors have included a generally round cross-section for receiving the conduit. However, in these designs, different parts of the ultrasound beam travel different distances through filling member between the transducer to the liquid under the test. The differences in ultrasound velocity between the filling member and the flow under test produce an ultrasound lens, and different parts of the flow will be sensed with different bias in the composite transit-time signal. In addition, a full field flow illumination of the liquid under test is compromised by differences in attenuation between the filling member, the conduit walls and the liquid under test. These differences result in uneven intensity distribution across the flow and thus, to substantially uneven sensitivity distribution inside the measuring channel. All such measurement errors may lead to measurement accuracy specification unacceptable for flow measurements in animal studies and human surgery.
Therefore, the need exists for an ultrasound sensing device and particularly a transit time flow sensor employing planar transmit and receive transducers in an acoustic path, wherein the acoustic path has reduced detrimental interference from passage of an acoustic signal through the conduit or an acoustic coupling. A further need exists for such a transit time flow sensor having a relatively low acoustic attenuation, thereby improving the available signal to noise ratio. The need also exists for a transit time flow sensor that can be employed in non-planar configurations without sacrificing full field illumination or measurement validity.